1. Field of the Invention
The present invention relates to a steering apparatus for a movable body such as a vehicle, and in particular, relates to a steering apparatus including a reaction force control device.
Priority is claimed on Japanese Patent Application No. 2004-274986, filed Sep. 22, 2004, the content of which is incorporated herein by reference.
2. Description of the Related Art
An electrical power steering apparatus is known as a steering apparatus for a vehicle. In an electrical power steering apparatus, a steering shaft connected to a steering wheel and a turning mechanism for turning steerable wheels are mechanically connected, and an electrical motor for assisting steering force is associated with the turning mechanism. In general, a driving torque command (drive current) for the electrical motor is controlled so that an assisting steering force is increased as the steering operation torque applied to the steering shaft increases.
Moreover, as such an electrical power steering apparatus, a steering system is known in which, in order to restrain irregular movement of the vehicle due to disturbance (e.g., crosswind), a vehicle movement (e.g., a yaw rate) is measured by a measuring means, a drive torque correction value for compensating for the vehicle movement is calculated, a drive torque command is calculated by subtracting the drive torque correction value from a drive torque base command that is set depending on the steering operation torque, and the electrical motor is controlled according to the drive torque command (see, for example, Published Japanese Patent No. 3229074).
In general in a related art, when a yaw rate is used to represent a vehicle movement, a control operation is performed so that the drive torque correction value is increased as the yaw rate increases. In other words, the driving torque of the electrical motor is controlled so that the assisting steering force by the electrical motor is decreased as the yaw rate increases. Accordingly, the drive torque correction value may be referred to as a reaction force with respect to the assisting steering force.
In the steering system constructed as explained above, even during, for example, a normal turning state in which a yaw rate appears, the drive torque correction value is generated so as to reduce the yaw rate, i.e., so as to control the vehicle toward a straightforward state, and thus the assisting steering force is reduced.
Accordingly, when the reaction force is controlled depending on the yaw rate, a steering stability is improved, and steering operation feel is also improved.
Moreover, as another control system to restrain irregular movement of the vehicle due to disturbance (e.g., crosswind), another steering system is also known in which a standard yaw rate is determined based on a steering angle and vehicle speed, an actual yaw rate is measured by a yaw rate sensor, a drive torque correction value is calculated which is required to make a deviation (hereinafter referred to as a yaw rate deviation) between the standard yaw rate and the actual yaw rate to be zero, a drive torque command is calculated by subtracting the drive torque correction value from a drive torque base command that is set depending on the steering operation torque, and the electrical motor is controlled according to the drive torque command (see, for example, Published Japanese Patent No. 3110892). As in the aforementioned case, because the assisting steering force is decreased as the drive torque correction value that is calculated based on the yaw rate deviation increases, the drive torque correction value may be referred to as a reaction force with respect to the assisting steering force.
By such a reaction force control operation depending on the yaw rate deviation, because the degree of understeer can be restrained and oversteer can be prevented, the vehicle stability can be improved.
However, when the reaction force is controlled depending on the yaw rate, although a steering operation feel may be improved, the vehicle stability (i.e., restraint of the degree of understeer and prevention of oversteer) cannot be sufficiently improved if the yaw rate is unexpectedly large or small (i.e., the yaw rate deviation is large).
On the other hand, when the reaction force is controlled depending on the yaw rate deviation, although the vehicle stability under a large yaw rate deviation can be improved, the steering operation feel may be degraded if such a control operation is carried out under a small yaw rate deviation.